Why? I mean, what are you trying to accomplish? You need more space, what other backups are you doing? How are the 6TB Lacie 2Bigs setup - are they a RAID0 of 2 3tb drives? Are you looking for faster disk performance than what you currently have?
Here's a cleaner method. Your current setup is 2 drives, with A sync'ing to B. What I would do is stop adding to A, put B somewhere safe & offline and add a new set of disks, C & D. Setup the same method you've been doing for A->B but for C & D. So there are 3 drives online: archive, live & backup. Archive has a mirror (B) that's tucked somewhere safe. If the archive disk that's plugged in dies, you pull your backup copy out (B), purchase a new disk and mirror it to the new drive.
Only thing I would consider is if the 2Bigs are setup as RAID0 I'd snag 2 large (8tb) externals instead. Use A as the archive, but copy B to 8tb(1), erase B & setup as the new Live disk. Finally, setup the 8tb(2) as the receiver of sync's from B. 8tb(1) is tucked away safe at this time. My intent is to consolidate your data onto single drives without any RAID. I do not recommend spanning any mirroring or RAID between external enclosures - you're a bad power supply of having all your data inaccessible, or worse, corrupted.
Unless you really want to get into the IT side of things, RAID should always be contained in a single enclosure. If you want to put a bit more into this, doing a multi-drive setup with
https://eshop.macsales.com/shop/Thunderbolt/External-Drive/OWC/ThunderBay-4-RAID5 and disks is pretty straight forward. The included copy of SoftRaid is pretty slick, but again, you're looking at a RAID5. If you can deal with USB3, it's cheaper, but everything is a trade-off.
## Edit
So with the current setup 2x 6tb disks in the 2Big, I'd actually slightly adjust it 6tb1 is your new 'archive' - copy the second 6tb drive to an external 8tb, format the second 6tb as your new 'live' and setup a new 8tb external as the sync to. If the enclosure or a drive in it dies, you have a copy of everything on other disks.
-Joe